Each database and driver has its own policies for how long to retain a connection before disconnecting with a timeout. This is not strictly controlled by Tableau. What you are noticing in version 8.0 is that we are doing a better job of retaining a connection for a longer period when it may serve as an advantage for quickly responding to followup queries, which can happen when a user begins interacting with a report after a long period of inactivity. However Tableau is still subject to the driver / database timeout policy, which forces Tableau to reconnect upon discovering that the connection is no longer usable. If you wish to alter this behavior, consider working with your Greenplum DBA, or if this is a limitation of the Greenplum system then I suggest you work with EMC to report this as a defect or perhaps a feature request.
In the meantime, if you have further evidence that this is a problem specific to Tableau then please contact our excellent Support team (firstname.lastname@example.org) so they may work with you to collect log files, determine the database server settings, etc.
I hope this helps,